Monophonic electronic musical instrument with variable filter

ABSTRACT

A monophonic electronic musical instrument comprising a keyboard circuit having key switches, a tone generator whose frequency is selectively determined by the key switches, a variable low-pass filter for filtering a tone signal from the tone generator, and a controlling DC voltage generator for producing a DC voltage to control the cutoff frequency of the filter. The DC voltage is so selected that the cutoff frequency is set in proportion to the frequency of the tone generator. Thus, flute tone signals of the same tone volume and color are obtained throughout the whole gamut of the instrument.

United States Patent Inventor Takehisa Amano I-lamamatsu-shi, Japan Appl. No. 842,547 Filed July 17, 1969 Patented Oct. 19, 1971 Assignee Nippon Gakki Seizo Kabuskiki Kaisha I-Iamamatsu-shi, Japan Priority July 19, 1968 Japan 43/50567 MONOPI-IONIC ELECTRONIC MUSICAL INSTRUMENT WITH VARIABLE FILTER 5 Claims, 10 Drawing Figs.

U.S.Cl 84/1.2l, 84/ 1.01 Int. Cl Gl0h l/02 Field of Search.. 84/1 .01

1.24, 1.19, 1.11 B, 1.11 E, 1.11 F, 1.11 L, 1.11 M

References Cited UNITED STATES PATENTS Primary Examiner-Milton O. Hirshfield Assistant ExaminerUlysses Weldon Att0meyCushman, Darby & Cushman ABSTRACT: A monophonic electronic musical instrument comprising a keyboard circuit having key switches, a tone generator whose frequency is selectively determined by the key switches, a variable low-pass filter for filtering a tone signal from the tone generator, and a controlling DC voltage generator for producing a DC voltage to control the cutoff frequency of the filter. The DC voltage is so selected that the cutoff frequency is set in proportion to the frequency of the tone generator. Thus, flute tone signals of the same tone volume and color are obtained throughout the whole gamut of 2,606,969 8/1952 Scott 333/18 theinstrument.

4 5 KEY- TONE BOARD GENERATOR PATENTEIIUET 19ml V 3','6l4.288

' SHEE? 10F 4 FIG; W PRIOR ART FIG.-2'

4 -KEY-' TONE BOARD GENERATOR F'LTER PATENTE Dnm 19 Ian v SHEET 2 UF 4 I 7 2 E q W mN G y- M 2 M m J b On 08 NM hmmmoz PATENTEnum 1 9 l97l 3. 6 1 4, 2 88 SHEET 3 0F 4 DRAIN-SOURCE RESISTANCE 4 FIG. 6

a .1: I E T 0-5 I I I I I I l I l I l o Vgs [v] FIG. 7

8 VARIABLE 6dB/oct- 1 I l l l I00 200 400 800 I600 3200 e400 FREQUENCY [Hz] I FIG. 8 (2 {3 KEY- TONE E {1 BOARD GENERATOR F'LTER [FREQWOLT j CONVERTER 6 MONOPIIONIC ELECTRONIC MUSICAL INSTRUMENT WITII VARIABLE FILTER The present invention relates to a monophonic electronic musical instrument having a filter whose frequency characteristic is shifted in accordance with the pitch of a tone signal from a tone generator. Heretofore a monophonic electronic musical instrument normallyemployed a filter whose circuit constant is fixed as shown in FIG. 1. Therefore, there was a defect that, when input signals of difierent tone pitch (frequency) were introduced, output levels were caused to vary and the contents of the harmonic frequencies were nonuniform through the tone range. Foi' example, when rectangular waves of different frequencies were introduced into a low-pass filter inorder to produce, for example, flute or wood sounds, the output level characteristics dropped at a range of higher pitches, dueto the characteristics of the low-pass filter, and this resulted in a great disadvantage when the instrument was playing a melody at a higher pitch. Further, since the output content of the harmonic frequency changes as the input frequency varies, there was a defect in that the tone color was caused to be distorted relative to the tonal heights.

The presentinvention is intended to eliminate the abovementioned defects and to provide a musical instrument wherein a variable filter is provided between a tone generator and an output amplifier so that the output characteristics thereof may be appropriately varied in accordance with the tone pitch.

The object of the present invention is to provide a monophonic electronic musical instrument comprising means for producing a controlling DC voltage in accordance with the operation of relative keys and a variable filter whose frequency characteristic is changed in accordance with said controlling voltage, said output characteristics of the filter being variable following the pitch variation of the tone signal so as to prevent the distortion of tone color and volume when the tone pitch changes.

Other objects and advantages of the invention will become apparent from the following description of the preferred embodiments and claims in conjunctionwith the accompanying drawings, in which FIG.! shows a fixed filter used in an exampleof a conventional circuit;

FIG. 2 is a block diagram showing an embodiment of the monophonic electronic musical instrument according to the present invention; a

FIG. 3 shows an example of a controlling voltage circuit employed in the keyboard circuit of FIG. 2;

FIG. 4 shows an exemplified circuit of a variable filter of the present invention;

FIG. 5 is a circuit diagram showing an actual example of the main portion of the block diagram given in FIG. 2;

FIG. 6 is a characteristic curve showing the relation between the gate voltage Vgs and the drain-source resistance Rds of the field effect transistor used in the variable filter in FIG. 5;

FIG. 7 shows the frequency characteristics of the variable filter in FIG. 5;

FIG. 8is ablock diagram showing another embodiment of the present invention;

FIG. 9 is a circuit diagram showing an actual example of the frequency-to-voltage converter in FIG. 8; and- FIG. 10 is a characteristic curve showing the relation between the frequency f and the output DC voltage V, of the frequency-to-voltage converter of FIG. 9. v

In FIG. 2, l is a keyboard circuit, 2 is a tone generator, 3 is a filter, 4-is an amplifier, 5 is aspeaker. The keyboard circuit 1 has two switches for each key. One of these switches is to control the frequency of the tone generator in relation .to the key operated and the other is, as shown by way of an example in FIG. 3, is to produce controlling DC voltage for the variable filter 3 when the key is depressed.

In FIG. 3, K,, K,, K,,---, K, represent transfer-contact switches each of which is actuated by the corresponding key. Resistors R R,, R,, R, are connected in series as illustrated. The point at which the related resistors are joined is connected to the normally open contact of the related switch. Therefore, when a desired key is depressed, only the switch that is connected to the key is actuated and thus the divided DC voltage at the junctions of the related resistorsis presented at a terminal T,. The values of resistors R R,, R,,--, R, are appropriately determined based on the provision of the frequency characteristics of the filter 3.

Therefore, a predetennined controlling voltage can be supplied to the control element of the variable filter so that the variable filter 3 has such characteristics as to make the cutoff frequency f, proportional to the input frequency f FIG. 4 shows an example of a variable filter employed in the musical instrument of the present invention, which is of a lowpass type consisting of a capacitor C and a field effect transistor used as a variable resistor. When the gate of the field effect transistor FET, that is, a terminal T,, is supplied with the controlling DC voltage, the value of resistance between the source and the drain of FET changes in accordance with the controlling voltage and thus the characteristics of the filter can be determined in accordance with the controlling voltage in such a way that there is substantially no change in the output signal levels produced at a terminal T and in the contents of harmonic frequencies despite the change in the frequencies of the input signals supplied at a terminal T In FIG. 4, BP is a terminal connected to a biasing power source and VR is a variable resistor for regulating the FET bias, which is provided when necessary.

FIG. 5 shows an actual example of the above-mentioned embodiment having a three octave range of the keyboard from C;( I31 Hz.) to C,( 1047 Hz.). Numeral 1 indicates a keyboard circuit, 3 represents a variable filter, and the rest represents a tone generator consisting of a variable frequency oscillator 20, two stages of the frequency dividers 21 and 22, a driver 23 and a keyer 24. The keyboard circuit includes two tapped resistors, the left one providing DC voltage for controlling the FET in the filter 3 and the right. one providing another DC voltage source for controlling the FET's and in turn the oscillation frequency of the oscillator 20. The oscillation frequency is determined in the range of 523 Hz. through 4l86 Hz. The divider 21 produces a half of the frequency derived from the oscillator 20. The divider 22 produces a further half of the frequency derived from the-divider 21. The'signals from the oscillator 20 and the dividers 21 and 22 are mixed and supplied to the keyer 24, which nonnally prevents the signal from passing to the filter but temporarily permits the signal to pass to the filter when a drive signal is supplied from a driver 23. The driver 23 receives an AC signal from the oscillator through capacitors, the right-hand resistors and a key switch in the key board circuit I, amplifies the AC signal, rectifies the AC signal into DC signal, and amplifies the DC signal to produce the drive signal. In this example, when a key is depressed, the oscillator starts to oscillate and the keyer starts to conduct, while the corresponding DC voltage is derived from the left-hand resistors in the keyboard circuit and is supplied to the filter to control its cutoff frequency. The drainsource resistance of the FET in the filter varies in accordance with the gate voltage Vgs supplied from the keyboard circuit, as shown in FIG. 6. In this case, when the key of C, note (131 Hz.) is depressed, the gate voltage is 2.1 volts, the resistance becomes 4 kilohms and the cutoff frequency of the filter is set at 200 Hz.; when the key of C, note (262 Hz.) is depressed, the gate voltage is l.7 volts, theFET resistance 2 kilohms and the cutoff frequency 400 Hz.; when the key of C, note (523 Hz.) is depressed, the gate voltage is I.l volts, the FET resistance I kilohm and the cutoff frequency 800 "2.; and when the key of C note 1047 Hz.) is depressed, the gate voltage is 0.1 volt, the FET resistance 0.5 kilohms and the cutoff frequency 1600 2. FIG. 7 shows the variation of the filter characteristic as mentioned above.

FIG. 8 illustrates another embodiment of the present invention, in which the modification resides in the point that, the controlling voltage is supplied to the filter 3 from the output side of a frequency-to-voltage converter 6 which is to produce the output DC signal whose voltage is dependent on the frequency of the tone generator signal. One example of the converter .circuit is shown in FIG. 9, with the frequency-tovoltage-relation being asshown by the curvein FIG. 10. The output signal of the oscillator (as in FIG. of the frequency f is supplied to the input terminal T of the converter and a DC voltage V, determined by the FIG. curve is taken out at the terminal T which in turn is connected to the gate of the FET of the variable filter 3 of the same construction as shown in FIG. 5 (this corresponds to the terminal T, in FlG.-4). With this example, the same control of the filter as described hereinbefore is carried out. The frequency-to-voltage converter may be of another type such as a discriminator used in a conventional FM broadcasting receiver set.

The advantage which results from the above construction of this invention is that, despite the change in tone pitch, the tone volume and the tone color remain substantially unchanged.

As many changes could be made in the above construction, and many apparently widely different embodiments of this invention could be made without departing from the scope of the claims it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A monophonic electronic musical instrument comprising:

a keyboard circuit including key switches,

a tone generator associated with said key switches and producing tone signals over a range of frequencies having harmonic components and being of a frequency selectively determined by said key switches,

a variable filter means having a frequency characteristic determined by a controlling DC voltage, said variable filter means being operatively connected to said tone determined in accordance with the corresponding frequency of said tone generator resulting when the respective key switches are actuated thereby providing the desired output characteristics of said variable filter.

3. A monophonic electronic musical instrument according to claim 1 in which'said means for producing said controlling DC voltage comprises a frequency-to-voltage converter connected to produce an output DC signal whose voltage is proportional to the frequency of the tone generator signal.

4. A monophonic electronic musical instrument according to claim I in which said variable filter comprises a field effect transistor as a controlling element to change the value of resistance between the source and drain of the transistor in accordance with the controlling DC voltage.

5. A monophonic electronic musical instrument comprising:

means for producing a controlling DC voltage in accordance with the operation of relative keys, and

a variable filter whose frequency characteristic is shifted in accordance with said controlling voltage,

said output characteristics of the filter being variable so as to prevent the distortion of tone color and volume when the tonal height changes,

said means for producing a controlling DC voltage comprising a frequency-towoltage converter arranged to produce an output DC signal having a voltage that is dependent on the frequency of the tone generator signal. 

1. A monophonic electronic musical instrument comprising: a keyboard circuit including key switches, a tone generator associated with said key switches and producing tone signals over a range of frequencies having harmonic components and being of a frEquency selectively determined by said key switches, a variable filter means having a frequency characteristic determined by a controlling DC voltage, said variable filter means being operatively connected to said tone generator for filtering said tone signals uniformly over the entire range of frequencies from said tone generator, and means for producing said controlling DC voltage in proportion to the frequency of said tone generator thereby insuring the uniform filtering of said tone signals over the entire range of frequencies.
 2. A monophonic electronic musical instrument according to claim 1 in which said means for producing said controlling DC voltage comprises a keyboard circuit having transfer-contact switches connected to the keys and resistors having values determined in accordance with the corresponding frequency of said tone generator resulting when the respective key switches are actuated thereby providing the desired output characteristics of said variable filter.
 3. A monophonic electronic musical instrument according to claim 1 in which said means for producing said controlling DC voltage comprises a frequency-to-voltage converter connected to produce an output DC signal whose voltage is proportional to the frequency of the tone generator signal.
 4. A monophonic electronic musical instrument according to claim 1 in which said variable filter comprises a field effect transistor as a controlling element to change the value of resistance between the source and drain of the transistor in accordance with the controlling DC voltage.
 5. A monophonic electronic musical instrument comprising: means for producing a controlling DC voltage in accordance with the operation of relative keys, and a variable filter whose frequency characteristic is shifted in accordance with said controlling voltage, said output characteristics of the filter being variable so as to prevent the distortion of tone color and volume when the tonal height changes, said means for producing a controlling DC voltage comprising a frequency-to-voltage converter arranged to produce an output DC signal having a voltage that is dependent on the frequency of the tone generator signal. 